Five more Australian MPs leave parliament after court's citizenship ruling

Australia faces a series of by-elections after the High Court ruled on Wednesday an 11th lawmaker must leave parliament in a test-case decision that prompted four more lawmakers to quit amid a widening citizenship crisis.

The full bench of the High Court unanimously agreed that Katy Gallagher was a citizen of a foreign power when she nominated for election to parliament in May 2016, and therefore incapable of being chosen. But the Home Office didn't register that she was no longer British until August 16.

Labor MPs Justine Keay, Josh Wilson, and Susan Lamb, plus independent MP Rebekha Sharkie have resigned from the House of Representatives in relation to dual citizenship violations stemming from Section 44 of the Constitution. All the politicians believed that the court ruling would also apply to their circumstances.

That would require an "insurmountable obstacle" to renouncing foreign citizenship, five justices said in a joint decision.

Gallagher's Senate seat will be replaced by another lawmaker from the Labor Party without an election.

With three lower house seats and one winnable Senate spot, Labor's rules crowd an electoral space that at first glance looks so spacious for two ALP candidates.

The dual citizenship saga has destabilised Australian politics since July 2017, at times threatening Turnbull's majority in the House of Representatives.

But it is resisting pressure from the Coalition to support the second part of the plan, which would radically simplify the tax system and put most Australian workers in the same tax bracket.

While the Turnbull government targets Shorten for his failure to force his MPs to resign sooner, the Labor leader has attempted to frame the looming contests - to be held as early as June - as a chance to cast judgment on the coalition government's May budget and 10-year company tax cut plan.

"The High Court has adopted the strictest possible reading", George Williams, professor and Dean of Law at the University of New South Wales in Sydney, told Reuters.